Halogenated ethenes, in the past, have had widespread use, inter alia, as degreasing agents in the metal processing industry and in textile cleaning. In the interim, these substances which are hazardous to health have become detectable in many places in groundwater and in soil.
Pollution incidents are principally treated by extraction of soil air or by stripping the groundwater aquifer with air. The exhaust air loaded with volatile components is then passed over activated carbon filters. In the course of this the pollutants are removed from the exhaust air stream by adsorption to the activated carbon. The loaded activated carbon can be regenerated and reused for cleaning up the exhaust air streams. If regeneration of the activated carbon is uneconomic, as in the case of the clean up of exhaust air streams with a low level of pollution, the activated carbon is incinerated together with the adsorbed pollutants.
In the incineration of chlorinated-hydrocarbon-loaded activated carbon, polychlorinated dioxins can form. The "17th regulation on the Federal German Air Pollution Control Act" therefore prescribes a dioxin limiting value of 0.1 ng/m.sup.3 of exhaust air for the thermal treatment of waste. Regeneration of activated carbon is uneconomic if only low concentrations of the pollutants to be removed are present in the exhaust air stream to be cleaned up and therefore the activated carbon has only a low loading on account of the adsorption isotherms. At low pollutant concentrations in the exhaust air to be cleaned up, on account of the low loading capacity, large amounts of activated carbon are required. The thermal disposal of this activated carbon has a considerable effect on the economic efficiency of the process, as a significant cost factor, in the case of soil and groundwater remediation.
In practice, the exhaust air stream is frequently only cleaned up in the initial phase of remediation using activated carbon filters at high pollutant concentrations. If the pollutant concentration decreases in the course of the remediation process below the statutorily prescribed limiting values of the German Air Pollution Control Regulations, as is usual in the extraction of soil air, the exhaust air is released to the atmosphere without clean up. This redistribution of the pollutants of soil, groundwater and soil air into the atmosphere is questionable for ecological reasons.
Soil, and especially groundwater, are highly protected materials. For reasons of environmental policy, therefore, attempts are made for the permanent remediation of chlorinated hydrocarbon pollutants, without causing a redistribution of the chlorinated hydrocarbons into the atmosphere. The clean up of exhaust air streams having a low level of pollution from the extraction of soil air is highly cost-intensive, however, since large amounts of activated carbon having a low level of pollution are produced which cannot be regenerated economically and must be disposed of thermally with high costs.
Under anaerobic conditions, in the case of pollution incidents caused by tetrachloroethene (PCE) and trichloroethene (TCE), owing to the activity of the autochthonous microflora of the soil, vinyl chloride (VC), which is a particular problem, is formed. Firstly, it is known to be carcinogenic and teratogenic, secondly, it is adsorbed to activated carbon only in extremely low amounts which are not industrially relevant. Chlorinated hydrocarbon pollution incidents having relatively high VC concentrations are therefore not considered to be remediable using the described methods of the current prior art.
For these reasons, biological processes suggest themselves as inexpensive alternative for cleaning up contaminated soil, groundwater and soil air. One possibility is, for example, to inject methane into the soil in order to enrich and activate there methane-oxidizing (methylotrophic) bacteria.
The biodegradation rate is dependent on the number of chlorine substituents and the presence or absence of oxygen. Highly chlorinated hydrocarbons, in particular PCE, are only biodegraded under anaerobic conditions, but lightly chlorinated hydrocarbons, in contrast, are predominantly biodegraded under aerobic conditions. Methane-oxidizing (methylotrophic) and aromatic-degrading bacteria are, for example, able to mineralize TCE, that is to convert it to carbon dioxide, water and chloride. The initial attack is made by an oxygenase. However, the oxygenase reaction forms a highly reactive epoxide which damages and deactivates the cell. The damage is caused by alkylation of cell proteins. The strain Pseudomonas putida F1 is deactivated very rapidly, for example, by the TCE conversion. The oxygenase activity decreases in the course of 20 minutes to 2% of the initial activity.
EP 0 447 862 B1 describes a process for the biological purification of gas streams contaminated with halogenated ethenes and/or halogenated butadienes, in particular from soil air. In this process, expensive growth substrates (auxiliary substrates) such as isoprene and/or butadiene are used. In practice, this process proved to be cost-intensive, since the bacteria used for regeneration must be supplied with relatively large amounts of isoprene. Since isoprene is a relatively readily degradable compound, the risk of an external infection by non-dehalogenating bacteria is very high and can only be prevented by complex sterile techniques which have considerable effects on the operating costs.
EP A 0 336 718 describes a further process for the microbiological degradation of trichloroethene using genetically modified microorganisms. Before using such a process for remediating pollution incidents, particular safety measures must be taken and laborious approval processes are required. Furthermore, problems with public acceptance cannot be excluded.
DE-A 3 326 057 describes a process for the biological clean up of exhaust air. Monochlorinated and dichlorinated alkanes, chlorobenzene and chlorotoluene can be mineralized by the microorganisms used. Degradation of halogenated ethenes is not described.
None of the processes is suitable for degrading PCE.
Starting from this, the object of the present invention is to specify a process for the microbial removal/elimination of halogenated ethenes, with which process a simple and complete degradation is possible.
The object is achieved by the characterizing features of claim 1. The subclaims indicate advantageous developments.